Consumer electronics devices are continually getting smaller and, with advances in technology, are gaining ever increasing performance and functionality. This is clearly evident in the technology used in consumer electronic products such as mobile phones, laptop computers, MP3 players and personal digital assistants (PDAs). Requirements of the mobile phone industry for example, are driving the components to become smaller with higher functionality and reduced cost. It is therefore desirable to integrate functions of electronic circuits together and combine them with transducer devices such as microphones and speakers.
The result of this is the emergence of micro-electrical-mechanical-systems (MEMS) based transducer devices. These may be for example, capacitive transducers for detecting and/or generating pressure/sound waves or transducers for detecting acceleration. There is a continual drive to reduce the size and cost of these devices through integration with the electronic circuitry necessary to operate and process the information from the MEMS through the removal of the transducer-electronic interfaces. One of the challenges in reaching these goals is the difficultly of achieving compatibility with standard processes used to fabricate complementary-metal-oxide-semiconductor (CMOS) electronic devices during manufacture of MEMS devices. This is required to allow integration of MEMS devices directly with conventional electronics using the same materials and processing machinery. This invention seeks to address this area.
Microphone devices formed using MEMS fabrication processes typically comprise one or more membranes with electrodes for read-out/drive deposited on the membranes and/or a substrate. In the case of MEMS pressure sensors and microphones, the read out is usually accomplished by measuring the capacitance between the electrodes. In the case of transducers, the device is driven by a potential difference provided across the electrodes.
FIG. 1 shows a capacitive microphone formed on a substrate 2. A first electrode 4 is mechanically connected to a membrane 6. A second electrode 8 is mechanically connected to a structurally rigid back-plate 14. During the manufacture of the MEMS device described above, the membrane 6 is formed using a sacrificial layer located between the membrane 6 and the second electrode 8. A back-volume 12 is formed using an etching process from below the substrate, known as a “back-etch”. The sacrificial layer between the membrane 6 and second electrode 8 is removed later in the process to leave the membrane 6 suspended and free to move.
A disadvantage of the process described above is that the back-etch is difficult to perform in an accurate manner when using a wet-etch or a dry-etch. In other words, it is difficult to obtain a consistent back-volume, particularly when performing a wet back-etch, since the sides of the back-volume tend to converge inwards as they approach the first electrode 4 and membrane 6, rather than being parallel as shown in the ideal scenario of FIG. 1. This tapering of the back-etch can alter the dimensions of the electrode 4 and membrane 6, and thereby change the operating characteristics of the microphone such as its frequency response and sensitivity.
It will also be appreciated that, in order to incorporate the transducers into useful devices, it is necessary to interface or couple them to electronic circuitry, which may either be located on the same substrate or a separate integrated circuit. However, this can lead to problems with interference, noise and parasitic capacitance and inductance.
Typically the membranes are thin, of the order of tenths of a micron, and can range in size from tens to thousands of microns. As a result, the devices can be fragile and may be damaged during singulation. Singulation is a process in which a substrate wafer on which the MEMS devices are fabricated is diced up so that only one device (or a group of devices) is found on each diced piece. This process is typically achieved by dicing the wafer with a high-speed rotating diamond blade. Alternatively the wafer may be cut up using a laser, or scribed and cleaved along a crystallographic axis. All of these dicing methods have associated problems when applied to MEMS structures.
During blade singulation the surface of the wafer is typically flooded with a lubricating coolant, usually water, that is meant to prevent the temperature of the wafer from becoming too high and to ensure the diamond blade stays in a safe operating range. This produces a slurry from the water and abraded pieces of wafer that may penetrate any open part of the MEMS structure and render it useless, as it is difficult to clean the slurry out at a later stage due to the small size of the singulated devices. Additionally, the lubricating coolant may be sprayed onto the wafer at high speed thus placing any delicate sensor structure under high mechanical stress and potentially damaging it.
Laser singulation is slightly cleaner than blade dicing but is more expensive. However, the heating produced by the cutting process may set up thermal gradients leading to areas of different thermal expansion in the sensor structures that may distort them and render them useless. Also the laser singulation process produces some residue that may clog any open structure and prevent the device from operating properly.
Finally, singulating the wafer by scribing and cleaving places extremely high mechanical stress on the wafer during the cleaving process and produces a large amount of debris that may damage the device as above.